1. Field of the Invention
This invention relates to a method of manufacturing a read/write head for writing information into a magnetic recording medium or reading information therefrom, and an erase head for erasing information written in the magnetic recording medium, and more particularly to a method of joining two magnetic members with a predetermined space kept therebetween.
2. Description of Related Art
Recently, a floppy disk drive of 3.5-inch 135TPI (Track per Inch) type has been in wide use. A magnetic head for such a drive is mainly of a tunnel erase and bulk type. This trend is anticipated to continue in the future. Floppy disks having a 2-Mega byte storage capacity are very popular, and will remain so in the future.
In this state, one of the market's main concerns is to obtain less expensive magnetic heads whose quality is maintained as usual.
Referring to FIGS. 1 to 3, a method of manufacturing a conventional magnetic head core with a two-piece slider will be described hereinafter. FIG. 1 is a perspective of a core viewed from above. FIG. 2 is a perspective view of the core in an inverted posture. FIG. 3 is a perspective of a core chip viewed from above.
The magnetic head including a two-piece slider as shown in FIGS. 1 to 3 has been widely employed up to now. The two-piece slider has large and small sliders 41 and 42, both of which sandwich a core chip 43 therebetween. The sliders 41 and 42 are cut from a large ceramic block by using a diamond wheel. Alternatively, they are obtained through powder compression molding or an injection molding process, and are joined together to hold the core therebetween. Both of the foregoing methods are unfortunately rather expensive.
Further, the large and small sliders 41 and 42 are bonded by a material such as an organic adhesive with the core chip 43 sandwiched therebetween. Therefore, since the large and small sliders and the core chip have different coefficients of friction on their confronting surfaces, there is a problem that lower surfaces 41a, 42a and 43a of the members 41, 42 and 43 are not on the same plane. This phenomenon is called "CS level difference" hereinafter. The CS level difference is variable in the order of approximately 20 .mu.m.
As is well-known, a recording/reproducing gap width (called "GD" hereinafter) is an important factor for determining the quality of the electromagnetic conversion characteristics. In FIG. 3, numeral 43b denotes GD. In order to assure a precise GD, a top surface of a core 44, defined by the large and small sliders 41 and 42 and the core chip 43, is ground. Several hundred cores 44 are arranged with their top surfaces upward and are ground simultaneously.
To prevent the cores 44 from having different GDs, it is important that a distance (43c shown in FIG. 3) between the lower surface of the core 44 and the lower end of the gap, i.e. back height of the core chip, should be uniform for all the core chips, that the CS level differences should be small, and that the cores 44 should be adhered to a GD processing plate in such a manner that an adhesive is applied in a uniform thickness.
A tolerance for the GD dimension should be practically within .+-.10 .mu.m of a proper value for the storage capacity of 2 MB. If a CS level is variable in a range of 20 .mu.m, it would become equal to the tolerance .+-.10 .mu.m for the GD dimension. For that reason, each core chip has to be ground in the order of 4 to 5 .mu.m with respect to the CS level differences. Thus, the core chips have to be processed after they are sorted according to their CS level differences.
Since the number of cores whose CS levels are variable near borders of the foregoing .+-.10 .mu.m range is relatively few, they should be stocked until they reach a predetermined quantity (e.g. several hundreds). In other words, such cores should be stocked and controlled as well as they are sorted.
Further, there are very few cores of which CS level differences and GDs are outside the foregoing .+-.10 .mu.m ranges. These cores have to be discarded, which would reduce an yield ratio of usable cores and prevent cost reduction.
Since stresses are applied to lower surfaces 43a, 41a and 42a of the core chips 43 and sliders 41 and 42 while they are hierarchically sorted according to the CS values, they would be damaged or cracked. Even when they do not apparently seem damaged or cracked, they would be prone to poor quality because of residual distortion therein or other factors.
When the large and small sliders 41 and 42 are bonded, by an adhesive, with the core chip 43 sandwiched therebetween, the surfaces to be bonded should be kept clean to a predetermined level to assure reliable bonding. Practically speaking, it is extremely difficult to provide a 100% bonding force. This is because there is a risk that the bonding surfaces might be stained since the bonding is manually carried out.
There is a rare instance in which joined surfaces are cracked to make cores fatally defective. There are some cores in which sliders 41 and 42 and core chips 43 are minutely displaced with respect to one another. This phenomenon is called "displacement". Since the displacement is very minute (e.g. 0.001 .mu.m to 0.3 .mu.m), it is very difficult to visually detect such a defect.
Specifically, if the phase shift is 0.01 .mu.m or less, the electromagnetic conversion characteristics of the core are not lowered remarkably. Such a core can probably pass an electromagnetic conversion characteristic test. If a magnetic head having such a core undergoes a medium wear test in which the medium is sought one million times, the magnetic material on the medium might be scraped away.
To overcome the foregoing problems related to the sliders and core chip bonded by the adhesive, there is a method to bond the sliders and core chip by glass. This method is free from the foregoing problems such as cracks and phase shift of bonded surfaces, but is prone to a problem that large and small sliders 61 and 62 should be formed with channel-shaped grooves 61a and 62a on their surfaces to be joined so as to receive bonding glass 64 in a minute space (1 .mu.m to 2 m) between the sliders 61 and 62 and the core chip 43. This process is very delicate and cumbersome.
Recently, a magnetic head having a one-piece slider as shown in FIG. 5 has been developed to overcome the foregoing problems of the magnetic head having the two-piece slider. In FIG. 5, reference numeral 15 denotes a one-piece slider 15 produced by the injection molding process. A bulk type core chip blank 17 is fitted into a long opening 16 on the one-piece slider 15, and is fixed therein with a melted glass rod 18. Thus, the magnetic head including the one-piece slider is free from the problems such as the CS level difference, damage of the portion between the core chip and the sliders, degraded electromagnetic conversion characteristics due to the displacement and scraping of the magnetic material from the medium, which are inevitable with magnetic heads with two-piece sliders.
However, there has arisen a new problem which is not significant when the core chip and sliders are bonded by the organic adhesive. This problem relates to a process of manufacturing core chips.
Referring to FIG. 6(A), a core chip will be manufactured as follows. As shown, a first magnetic material 21 for read/write and erase heads and a second magnetic material 22 are placed on a jig in a manner such that their contacting surfaces 21a and 22a confront each other via at least one spacer partially inserted therebetween so as to keep a predetermined gap g. Next, cylindrical glass rods 25 are melted to join the two magnetic materials 21 and 22 to form a bar material 27 as shown in FIG. 6B. Alternatively, as shown in FIG. 7(A), a silicon dioxide layer 23 (called "SiO.sub.2 layer 23"), whose thickness corresponds to approximately three quarters of the gap width, is formed on the surface 21a of the first magnetic material 21 by a vacuum evaporation process or a sputtering process. Then, a glass layer is sputtered on the SiO.sub.2 layer 23 in the width of approximately a quarter of the gap width g. Thereafter, the cylindrical glass rod 25 is melted to obtain the bar material 27 for the read/write and erase heads. This process for forming the bar material 27 is called "the first glass bonding process".
Referring to FIG. 8, head materials 6 and 7 which include track regulating grooves 6a and 7a thereon and are of level height are arranged in a manner such that first magnetic materials 1 of the head materials 6 and 7 confront each other with a minute gap 8 (called "center shield") kept therebetween by a thin glass plate 9 which is inserted in the lower part of the minute gap 8. The head materials 6 and 7 confront each other so as to let track regulating grooves 6a and 7a have a predetermined positional relationship. At this time, the head materials 6 and 7 are temporarily bonded at their lengthwise ends by a non-organic heat-resistant adhesive 10 such as ARON CERAMIC (trade name). Thereafter, a glass rod 11 having an oval section is placed on the head materials 6 and 7, and is heated and melted, thereby forming a core material 12. This process is called "the second glass bonding process".
The core material 12 is then cut and ground at a right angle along its length to obtain a bulk type core chip blank 17. This core chip blank 17 is inserted into the long opening 16 of the one-piece slider 15 which has been injection-molded. Thereafter, a cylindrical glass rod 18 is placed on the one-piece slider 15, and is melted to join the core chip blank 17 and the one-piece slider 15. This process is called "the glass molding process". When the glass is used, there are three process as described above.
In the first and second glass bonding processes and the glass molding process, it is assumed that the glass rods 25, 11 and 18 have working points Tw(.degree. C.) and softening points Ts(.degree. C.). Specifically, the glass rod 25 has Tw1 and Ts1, the glass rod 11 has Tw2 and Ts2, and the glass rod 18 has Tw3 and Ts3. The following relationships should be observed: Tw1-Ts1.apprxeq.200, Tw2-Ts2.apprxeq.200, Ts1.gtoreq.Tw2, and Ts2.gtoreq.Tw3. In the sputtering process shown in FIG. 7, the sputtering glass to form a glass layer 24 should have the softening point Ts which is equal to or slightly higher than the softening point Ts1 of sealing glass 25. Otherwise, the glass layer cannot be in close contact with the second magnetic material 22.
This is because if the glass used in the first glass bonding process has the softening point Ts1 which is lower than the working point Tw2 of the glass in the second glass bonding process, the glass 26 in the first glass bonding process and the glass layer 24 are softened in the second glass bonding process. This would change the gap width g.
Similarly, when the glass 17a in the second glass bonding process has the softening point Ts2 which is lower than the working point Tw3 of the glass used in the glass molding process, the glass in the second glass bonding process would be softened in the glass molding process. This would lead to a phenomenon in which the core chip for read/write and erase heads might be bent at the center shield thereof in the shape of an inverted V as shown in FIG. 9.
A difference between the working point Tw and the softening point Ts of a glass rod depends upon the kind of glass. In the case of low temperature glass, the difference is between 100.degree. C. and 200.degree. C. while in the case of high temperature glass, the difference is between 200.degree. C. and 250.degree. C. On the basis of the foregoing relationship, there should be a difference of approximately 400.degree. C. between the working point Tw1 of the glass in the first glass bonding process and the working point Tw3 of the glass in the glass molding process. Usually, a core chip of a magnetic head in a floppy disk drive is made of manganese zinc ferrite. The manganese zinc ferrite can endure a maximum temperature of 900.degree. without degrading its magnetic characteristics. Therefore, when Tw1 is set to maximum 900.degree. C., Tw3 inevitably becomes as low as 500.degree. C. The glass with 500.degree. C. Tw3 has 400.degree. C. or less Ts3, which means that this glass is easily affected by ambient conditions. Therefore, the glass cannot be used in a device such as a floppy disc drive which might be used under severe ambient conditions of high temperatures, high humidity, or extremely low temperatures.
As described above, the first method using the organic adhesive to bond the sliders and core chip is prone to problems of less reliable bonding, sorting work due to the CS level differences, and increase of the manufacturing cost. The second method using the glass comprises three steps, i.e. the first and second glass bonding processes and the glass molding process, as well as a step of making core chips, and selection of the glass to be used is difficult because softening points thereof are relatively limited.